The present invention concerns an optimized color display device.
The visual appearance at each point in a color image is entirely defined by three independent parameters: luminance, hue and saturation, the last two determining the color of the point concerned. Perception by the human eye of variations of luminance is quite different from that of variations of color in space and time.
Traditional display systems do not provide for separate control of the luminance and the color; consequently, they are poorly adapted to our different perceptions of luminance and color.
There are two ways of forming a color image, the first known as "additive" and the second as "subtractive".
Systems using the additive method modulate the respective luminous intensities of three separate beams each of constant color, then superimpose these beams. These three distinct colors are known as primary colors; they are often, for example, red, green and blue. Several techniques for taking into account the colors of neighboring points in an image are known in order to perform additive mixing: spatial average of neighboring elements, time average, spatial superimposition.
The visual appearance of each point in the image is controlled by modifying the intensities of each of the three primary beams; the ratios of these intensities then determine the visual color of the point; the sum of these three intensities determines the luminance of the point. To modify only the luminance of a point, without modifying its color, the luminous intensities of the three primary beams must be modified. Similarly, to modify only the color, without modifying the luminance, the luminous intensities of the three primary beams must again be modified.
In systems using the subtractive method, the three parameters modulated are the respective levels of transmission of three bandpass filters of fixed color traversed successively by a beam which is originally white. These three distinct colors are, for example, the complementary colors of the three primary colors mentioned earlier: cyan, magenta and yellow.
We control the visual appearance of each point in the image by modifying the transmission level of each of the three filters. To modify only the luminance of a point, without modifying its color, the level of transmission of the three filters must be modified. Similarly, to modify only the color, without modifying the luminance, the level of transmission of the three filters must again be modified. Another disadvantage of subtractive systems is that the maximum value of the transmitted intensity is limited by the fact that each filter always absorbs at least one spectral band.
The control of color image systems, whether additive or subtractive, therefore consists in using several spectral bands at predetermined positions in the visible spectrum, modulating them in varying proportions, then adding them to or subtracting them from the incident beam of light from the light source. These systems do not enable direct control of the color without modifying the luminance, nor of the luminance without modifying the color. One of the consequences is that the definitions of the luminance and the color of images points are the same, in other words, for a given image, the maximum number of luminance components in image points is the same as the maximum number of color components in image points, or the maximum spatial frequency of modulation of the luminance, at constant color, that the system can reproduce correctly is equal to the maximum spatial frequency of modulation of the color, at constant luminance, that the system can reproduce correctly.
Unfortunately, human visual performance, in terms of spatial frequencies in particular, is not very compatible with the techniques used in known image synthesis systems: visual acuity is much keener for luminance than for color. Known systems are not therefore well adapted to the characteristics of human vision.